Which of the following best describes the role of water in photosynthesis?

• A. Water acts as the primary energy source for the Calvin cycle.
• B. Water is converted into glucose during the light-independent reactions.
• C. Water directly absorbs sunlight to produce ATP.
• D. Water donates electrons to replace those lost by chlorophyll during the light-dependent reactions.

Answer: (D)

Verified step by step guidance

1. Understand the process of photosynthesis, which consists of two main stages: the light-dependent reactions and the Calvin cycle (light-independent reactions). Water plays a critical role in the light-dependent reactions.
2. Recall that during the light-dependent reactions, sunlight excites electrons in chlorophyll molecules within the thylakoid membranes of chloroplasts. These excited electrons are transferred to the electron transport chain.
3. Recognize that chlorophyll molecules lose electrons during this process, and these electrons must be replaced to maintain the cycle of energy transfer. Water serves as the source of replacement electrons.
4. Break down the role of water: Water molecules are split in a process called photolysis, which occurs in the presence of light and enzymes. This splitting produces electrons, protons (H⁺ ions), and oxygen gas (O₂).
5. Conclude that the electrons donated by water replace those lost by chlorophyll, ensuring the continuation of the light-dependent reactions. This is the key role of water in photosynthesis.